3,197 research outputs found
Critical properties of the Kitaev-Heisenberg model
We study critical properties of the Kitaev-Heisenberg model on the honeycomb
lattice at finite temperatures which might describe the physics of the quasi
two-dimensional compounds, NaIrO and LiIrO. The model undergoes
two phase transitions as a function of temperature. At low temperature, thermal
fluctuations induce magnetic long-range order by order-by-disorder mechanism.
Magnetically ordered state with the spontaneously broken symmetry
persists up to a certain critical temperature. We find that there is an
intermediate phase between the low-temperature ordered phase and the
high-temperature disordered phase. The finite-sized scaling analysis suggests
that the intermediate phase is a critical Kosterlitz-Thouless phase with
continuously variable exponents. We argue that the intermediate phase has been
actually observed above the low-temperature magnetically ordered phase in
NaIrO, and likely in LiIrO.Comment: 5 pages, 6 figure
Review of Is utopianism a threat to our grand experiment in democracy? Review of Ameritopia: The Unmaking of America by Mark R. Levin New York: Threshold Editions, 2012. $26.99 ISBN: 978-1439173244
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Parental investment and child health in a Yanomamö village suffering short-term food stress
The 1998 El Niño significantly reduced garden productivity in the
Upper Orinoco region in Venezuela. Consequently, parents were forced to
allocate food carefully to their children. Nutrition data collected from village
children combined with genealogical data allowed the determination of which
children suffered most, and whether the patterns of food distribution
accorded with predictions from parental investment theory. For boys, three
social variables accounted for over 70% of the variance in subcutaneous fat
after controlling for age: number of siblings, age of the mother’s youngest
child, and whether the mother was the senior or junior co-wife, or was
married monogamously. These results accord well with parental investment
theory. Parents experiencing food stress faced a trade-off between quantity
and quality, and between investing in younger versus older offspring. In
addition, boys with access to more paternal investment (i.e. no stepmother)
were better nourished. These variables did not account for any of the
variance in female nutrition. Girls’ nutrition was associated with the size of
their patrilineage and the number of non-relatives in the village, suggesting
that lineage politics may have played a role. An apparent lack of relationship
between orphan status and nutrition is also interesting, given that orphans
suffered high rates of skin flea infections. The large number of orphans being
cared for by only two grandparents suggests that grooming time may have
been the resource in short supply.
Introductio
Implicit imitation in multiagent reinforcement learning
Imitation is actively being studied as an effective means of learning in multi-agent environments. It allows an agent to learn how to act well (perhaps optimally) by passively observing the actions of cooperative teachers or other more experienced agents its environment. We propose a straightforward imitation mechanism called model extraction that can be integrated easily into standard model-based reinforcement learning algorithms. Roughly, by observing a mentor with similar capabilities, an agent can extract information about its own capabilities in unvisited parts of state space. The extracted information can accelerate learning dramatically. We illustrate the benefits of model extraction by integrating it with prioritized sweeping, and demonstrating improved performance and convergence through observation of single and multiple mentors. Though we make some stringent assumptions regarding observability, possible interactions and common abilities, we briefly comment on extensions of the model that relax these
Is Uber a substitute or complement for public transit?
How Uber affects public transit ridership is a relevant policy question facing cities worldwide. Theoretically, Uber’s effect on transit is ambiguous: while Uber is an alternative mode of travel, it can also increase the reach and flexibility of public transit’s fixed-route, fixed-schedule service. We estimate the effect of Uber on public transit ridership using a difference-in-differences design that exploits variation across U.S. metropolitan areas in both the intensity of Uber penetration and the timing of Uber entry. We find that Uber is a complement for the average transit agency, increasing ridership by five percent after two years. This average effect masks considerable heterogeneity, with Uber increasing ridership more in larger cities and for smaller transit agencies.
Document type: Articl
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